There has recently been an increasing tendency toward adopting an aseptic filling system as a method for filling contents in a container capable of preventing deterioration of flavor caused by heating contents such as a drink. In a case where the container is a plastic bottle such as a PET bottle in the aseptic filling system, there is generally adopted a method according to which hot water or a heated sterilizer such as peracetic acid, hydrogen peroxide or alkali is injected from a mouth portion of the bottle onto an inner surface of the bottle before filling contents such as a drink thereby to sterilize at least the inner surface of the bottle. In this case, there is the problem that the bottle must have resistance (strength) to heat corresponding to a sterilizing temperature and, if the sterilizing temperature exceeds the heat resisting temperature, thermal shrinkage occurs in the bottle resulting in deformation of the bottle.
Particularly in recent years, there is an increasing social demand for reducing weight of a bottle. As a bottle becomes lighter, there is a tendency to decrease in thickness of a trunk portion of the bottle and, therefore, the problem of occurrence of thermal shrinkage has become prominent.
As a method for sterilizing the inner surface of the bottle without causing thermal shrinkage in a non-heat resistant plastic bottle such as a PET bottle, there has generally been adopted a method for sterilizing an inner surface of a bottle by injecting hot water or a heated sterilizer at a temperature below 70° C., avoiding a temperature range from 70° C. to 95° C. which is a normal sterilizing temperature and a temperature which causes thermal shrinkage. Japanese Patent Application Laid-open Publication No. 2003-181404, for example, discloses a method for sterilizing a bottle by injecting a heated sterilizer from a first nozzle onto an inner surface of an inverted bottle and also storing a predetermined amount of the sterilizer in a mouth portion of the bottle, and stirring and raising this stored sterilizer by a sterilizer injected from a second nozzle whereby a part of the sterilizer staying in a mouth portion of the bottle is scattered over the inner surface of the bottle and the sterilizing effect is improved. The temperature of this sterilizer is 63° C. which is below the lower limit of the temperature which causes thermal shrinkage and sterilizing time is 10 seconds. Although this method has no problem of causing thermal shrinkage, there arises the problem that, since the sterilizing temperature is so low that the sterilizing time is prolonged and production efficiency thereby is decreased.
As another method for sterilizing a non-heat resistant plastic bottle without causing thermal shrinkage, there is known a method according to which the bottle is formed with a heat resistant plastic resin which does not cause thermal shrinkage at a normal sterilizing temperature within a range from 70° C. to 95° C. This method however has the problem that the heat resistant plastic resin is expensive and, besides, it requires many processes for forming the bottle with the result that the cost for manufacturing the bottle increases significantly.
Japanese Patent Application Laid-open Publication No. Hei 7-187149 discloses a method in which hot water is supplied to at least an inner surface of a plastic bottle and simultaneously cooling water which is of a lower temperature than the hot water is supplied to an outer surface of the bottle. According to this method, sterilizing can be completed without causing thermal shrinkage even in a case where high temperature water of e.g., 85° C. or over is injected into the inside of a non-heat resistant bottle. Likewise, there is disclosed a method according to which steam is injected in the inside of a bottle and simultaneously cooling water which is of a lower temperature than the steam is supplied onto the outer surface of the bottle to avoid occurrence of thermal shrinkage. Since, however, these bottle sterilizing methods cool the outer surface of the bottle with cooling water simultaneously with heating of the inner surface of the bottle with hot water or steam, difficulty arises in transmitting heat which is necessary for sufficiently sterilizing the inner surface of the bottle with the result that sterilizing time is prolonged and production efficiency is decreased and, further, as the sterilizing time is prolonged, a larger production line becomes necessary and the cost and space of equipment increase. Further, according to these methods, since the outer surface of the bottle is cooled with cooling water simultaneously with sterilizing of the inner surface of the bottle, in a case where it is necessary to sterilize the outer surface of the bottle also, the outer surface must be sterilized again with hot water with the result that two processes are required which necessitates further enlargement of the equipment.
The present invention has been made to solve the above described problems of the prior art method for sterilizing a plastic bottle in the aseptic filling system. It is an object of the invention to provide a method for sterilizing a plastic bottle capable of sterilizing a plastic bottle by a simple process and using heated sterilizing fluid such as hot water or a heated sterilizer at a normal sterilizing temperature within a range from 70° C. to 95° C. without causing deformation of the bottle due to thermal shrinkage.